This invention relates generally to communication networks and, more particularly, to devices known as bridges, connected to two or more networks and providing a convenient mechanism for transferring data packets between one network and another. Even more particularly, the invention relates to techniques for minimizing latency or time delay caused by operation of the bridge. The bridge latency is the time that elapses between receipt of the beginning of a data packet from one network to which the bridge is connected and transmission of the beginning of the same data packet onto another network to which the bridge is connected.
Bridges include a buffer memory for the temporary storage of data packets that cannot be immediately forwarded to another network. Use of the buffer memory necessarily increases the bridge latency, by an amount that varies with packet size. There are, however, times during operation of the bridge when transmission of a packet could begin before the entire packet had been received, and the packet could be passed straight through from one network to another, with practically no delay. This mode of operation is referred to as cut-through operation, and some bridges provide for operation in this mode.
Bridges providing for cut-through operation prior to the present invention relied on one of two implementation schemes: one that used starting or ending delimiters in each data packet, or one that used packet length information in a header associated with each packet. Both schemes have drawbacks. The use of delimiters requires additional buffering capacity, to the extent of one additional bit of storage for each word of data, and sometimes more if data alignment information is also required. The principal difficulty with using packet length information is that not all received packets will contain length information. Moreover, cut-through operation can only begin after the packet length has been identified and processed. Thus, if the packet length is not at the very beginning of the received packet there is always going to be an unavoidable latency.
It will be appreciated from the foregoing that there is a need for a different approach to cut-through operation of bridges, preferably one that avoids the disadvantages of the prior art. The present invention satisfies this need.